Re: [PATCH] PCI: Make sure the bus bridge powered on when scanning bus

[Yicong Yang <yangyicong@hisilicon.com>]

On 2020/9/18 5:07, Bjorn Helgaas wrote:
> [+cc Mika, Rafael, Peter]
>
> On Wed, Jul 29, 2020 at 07:30:23PM +0800, Yicong Yang wrote:
>> When the bus bridge is runtime suspended, we'll fail to rescan
>> the devices through sysfs as we cannot access the configuration
>> space correctly when the bridge is in D3hot.
>> It can be reproduced like:
>>
>> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/0000:81:00.1/remove
>> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/pci_bus/0000:81/rescan
>>
>> 0000:80:00.0 is root port and is runtime suspended and we cannot
>> get 0000:81:00.1 after rescan.
>>
>> Make bridge powered on when scanning the child bus, by adding
>> pm_runtime_get_sync()/pm_runtime_put() in pci_scan_child_bus_extend().
>>
>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
>> ---
>>  drivers/pci/probe.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 2f66988..5bb502b 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -2795,6 +2795,14 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>>  
>>  	dev_dbg(&bus->dev, "scanning bus\n");
>>  
>> +	/*
>> +	 * Make sure the bus bridge is powered on, otherwise we may not be
>> +	 * able to scan the devices as we may fail to access the configuration
>> +	 * space of subordinates.
>> +	 */
>> +	if (bus->self)
>> +		pm_runtime_get_sync(&bus->self->dev);
> I think if we do this, we should be able to remove the call from
> pci_scan_bridge() added by d963f6512e15 ("PCI: Power on bridges before
> scanning new devices"), right?
>
> The reason we need it here is because there are two paths to
> pci_scan_child_bus_extend() and only one of them calls
> pm_runtime_get_sync():
>
>   pci_scan_bridge_extend
>     pm_runtime_get_sync
>     pci_scan_child_bus_extend
>
>   pci_scan_child_bus
>     pci_scan_child_bus_extend
>
> If we move the pm_runtime_get_sync() from pci_scan_bridge_extend() to
> pci_scan_child_bus_extend(), both paths should be safe.

A bit different, I think. The issue I met is a bit different from Mika, as
we go through different sysfs files. Think about rescanning device under a 
root port,

when echo 1 > /sysfs/bus/pci/devices/${RootPort}/rescan:

rescan_restore()
  pci_rescan_bus(pdev->bus) /* we will rescan the root bus */
    pci_rescan_child_bus()
      pci_scan_child_bus_extend()  /* we cannot wake up the bus bridge here as is on the root bus */
        pci_scan_bridge_extend() /* we have to wake up the root port here */

when echo 1 > /sysfs/bus/pci/devices/${RootPort}/pci_bus/${PciBus}/rescan:

rescan_restore()
  pci_rescan_bus(bus) /* we will rescan the bus of the root port */
    pci_rescan_child_bus()
      pci_scan_child_bus_extend() /* we can wake up the bus bridge - root port here */

As different bus is rescanned, so it'll have problem without patch d963f6512e15.


>
>>  	/* Go find them, Rover! */
>>  	for (devfn = 0; devfn < 256; devfn += 8) {
>>  		nr_devs = pci_scan_slot(bus, devfn);
>> @@ -2907,6 +2915,9 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>>  		}
>>  	}
>>  
>> +	if (bus->self)
>> +		pm_runtime_put(&bus->self->dev);
> I would probably do this:
>
>   struct pci_dev *bridge = bus->self;
>
>   if (bridge)
>     pm_runtime_get_sync(&bridge->dev);
>   ...
>   if (bridge)
>     pm_runtime_put(&bridge->dev);

Sure.

Regards,
Yicong


>
>>  	/*
>>  	 * We've scanned the bus and so we know all about what's on
>>  	 * the other side of any bridges that may be on this bus plus
>> -- 
>> 2.8.1
>>
> .
>